This invention relates to golf balls and more particularly to golf balls having indentations or dimples formed on the surface which are evenly and uniformly distributed so that the ball has two perpendicular axes of symmetry. The geometric shapes forming the dimple constraining pattern for these golf balls do not have apices or centers at the poles.
In the manufacture of golf balls, regardless of whether the golf ball is of the solid, two-piece or three piece variety, two essentially hemispherical mold cavity halves are brought together in a molding press and the outer surface of the ball is formed. Where these mold halves meet is known as the "seam line" of the ball. Manufacturing constraints require that the seam line be free of any dimples. This dimple free great circle which is created on the surface of the ball presents two major problems to the golf ball manufacturer.
The first problem is of a purely aesthetic nature. The lack of dimples at the seam line appears to be a manufacturing flaw or defect in the eyes of the consumer. The increased fret area or space between the dimples in this area can give the illusion that the ball is not spherical. This illusion is further enhanced by the fact that the dimples of the constraining pattern of all golf balls either meet at the poles in apices or has the pole as the center of one of its constraining figures. U.S. Pat. No. 4,932,664 discloses a golf ball having a dimple placement on the surface of the sphere in order to minimize the appearance and effect of an unbroken seam line. Since the constraining pattern has a polar dimple which is the center of a pentagon and the pattern is radially repeated five times about this polar dimple, the pattern is not completely effective.
The second and more severe problem is that by having only one dimple free great circle, the aerodynamic performance of the ball is affected. Smooth, undimpled areas on a golf ball have increased aerodynamic drag. If a ball is hit in such a manner that this dimple free great circle is exposed during the majority of the flight of the ball, the ball will fly significantly differently than if this dimple free area is exposed only occasionally during the flight of the ball.
This difference in aerodynamic performance has led the United States Golf Association (USGA) to establish a rule governing the flight performance of a golf ball. This rule, known as the "symmetry rule", establishes very tight performance specifications on the differences in performance of the golf ball when struck on the equator (seam line) such that rotation of the ball is about an axis through the equator (referred to as pole over pole rotation) versus when the ball is struck on the equator such that rotation of the ball is about an axis through the poles of the ball (referred to as poles horizontal rotation). If the ball fails to meet the criteria of the symmetry rule it is taken off the list of balls approved for tournament play. This naturally has a disastrous effect on the sales of the product and requires that the manufacturer make changes in the product so that it will pass the "rule". These changes involve expensive tooling changes and even though these changes are made, the ball will not be retested for a minimum of six months.
The possibility of being removed from the approved list of the USGA has caused the manufacturers to seek dimple patterns which will meet the aerodynamic performance criteria of the "symmetry rule". A number of patented dimple patterns involving multiple parting lines or dimple free great circles has resulted. U.S. Pat. Nos. 4,560,168; 4,762,326; 4,765,626; 4,772,026; and 4,948,143 all describe patterns with multiple parting lines. These patents teach 6, 7, 3, 6, and 4 parting lines or dimple free great circles respectively.
While each of the patents referenced in the previous paragraph offer a solution to the passage of the symmetry rule it has long been known and is taught in U.S. Pat. No. 4,141,559 that it is most advantageous to have no circumferential pathways which do not intersect dimples if the distance performance is to be maximized.